Depends on your Motherboard BIOS that determines how much of the APU Integrated Graphics uses System RAM Memory as vRAM.
Maybe with the newer AMD APUs it can allot more then 2GBs of System RAM Memory.
You need to check your BIOS Settings to see what is the maximum vRAM using System RAM Memory that can be assigned to your APU.
NOTE: Searching the internet I see that Asus has increased the vRAM of the 3400G to 3GB Maximum in BIOS. According to this Tom's Hardware thread the reason why you can't assign more than a certain amount of vRAM to a APU: